Nuclear reactors, and particularly pressurized water-cooled nuclear reactors, comprise a vessel of generally cylindrical shape having a vertical axis and enclosing the reactor core, which consists of fuel assemblies with a prismatic shape Juxtaposed and disposed vertically. A primary circuit ensures the circulation and cooling of the pressurized water forming the primary coolant of the nuclear reactor. The primary circuit comprises at least one loop, and generally three or four loops, which are in communication with the internal volume of the vessel at their ends. The fuel assemblies of the core are disposed and maintained in partitioning surrounding the core inside the vessel. The vessel is closed at its upper part by a detachable cover which permits access to the inside of the vessel, for example to carry out the reloading of the core with fuel assemblies.
Nuclear reactors, and particularly pressurized water-cooled nuclear reactors, comprise means for measurement making it possible to obtain the value of the neutron flux in the core of the nuclear reactor and the spatial distribution of this neutron flux while the reactor is running.
These means for measurement comprise detectors of various types which can be introduced into the core of the nuclear reactor and which constitute the internal instrumentation of the core.
The detectors may consist, for example, of fission probes, gamma thermometers, aeroballs or hydroballs. These detectors are introduced into a certain number of fuel assemblies in such a way as to allow a determination of the spatial distribution of the flux in the interior of the whole core.
The detectors may be permanently present in the core and, in this case, the instrumentation is said to be fixed or, on the contrary, these detectors may be introduced into the core when they are being used for measurements, and the instrumentation is then said to be mobile.
In the first case, the flux measurements are used directly for carrying out surveillance of the core.
In the second case, the surveillance of the core is carried out using the data provided by other sensors, for example multi-stage fission chambers arranged on the outside of the core and devices for determining the position of the control clusters in the core.
In this latter case, the mobile internal instrumentation of the core has the sole function of carrying out a periodic readjustment of the data supplied by the external instrumentation and of detecting, before the start-up of the reactor, any mistakes that might have been made during fuel loading operations.
The internal instrumentation of the core also comprises tubes or "thimbles" which are used for the protection and guidance of the measuring detectors during their displacement from the instrumentation control room to the fuel assemblies. The thimbles pass through the wall of the nuclear reactor vessel by means of feed-through sleeves disposed in general in the lower part of the vessel or over the cover. The thimbles must be withdrawn from the fuel assemblies before any fuel discharge and reloading of the reactor with fuel assemblies is carried out.
In the instrumentation control room, facilities are installed for processing the measurements provided by the detectors and, when they are mobile detectors, devices are installed to provide for their movement inside the thimbles, in the direction of introduction into the core or in the direction of withdrawal from the core.
The instrumentation of nuclear reactors, whether fixed or mobile, has certain disadvantages.
It is first of all necessary to reserve considerable space in the reactor building to house premises intended to receive the measuring equipment and the control devices for the instrumentation and the passageways for the thimbles from the premises to the reactor vessel.
In addition, it is necessary to withdraw the thimbles before each of the core refueling operations, over a length at least equal to the height of the core, which requires the provision of a sufficient clearance in the control room, the volume of which thus has to be increased still further.
When the feed-through sleeves in the wall of the vessel permitting the introduction of the thimbles are fixed to the base of the vessel, there is a risk that the primary fluid contained in the vessel will leak in the case of failure of one or more of the feed-through sleeves.
In the case where the internal instrumentation of the core passes through the wall of the vessel at its upper part formed by the cover, the upper internal items of reactor equipment may have a relatively complex structure, and the servicing times and the radiation doses received by the operational staff during the withdrawal and replacement of the thimbles, before and after refueling of the core, may be increased.